We evolved on very close terms with darkness, says Eric Harrison, and we've forgotten its soft and nurturing qualitiesIt is natural for us to think dualistically by using pairs of opposites such as good and bad, light and dark, body and mind, male and female, growth and decay. These have an appealing symmetry and typically feel both profound and true. Some dualities are less than accurate - communism is not really the opposite of capitalism - but they help us define something by what it is not. Certain dualities, such as black and white, are genuine opposites. Others, such as night and day, can't be separated at all, since they are the opposing poles of a single, alternating system.

Our most familiar duality, operating every second of our day, is good and bad. The Buddha said that it is a prerequisite for clear thought to notice the pervasiveness of our judging faculty. Everything we actually notice, be it a thought, sensation or emotion, comes wrapped up with a feeling and an evaluation. We either like it or dislike it, we find it pleasant or unpleasant to some degree. We can consciously modify these automatic judgements, but we have no chance of ever seeing anything "just as it is" with the enlightened eye of a disinterested observer. Perfect objectivity or "pure consciousness" free of value judgement is a myth from the past.

We judge literally everything, however trivial. Researchers have demonstrated that if you show people a list of nonsense words, they will automatically prefer some over others. Nor is this an inconsequential act. A nonsense word they find unpleasant will create measurable changes in their physiology. Too many unpleasant words will stimulate emotional circuits in the brain and start to disturb cognitive functions such as short term memory. Psychologists call this "approach or withdrawal" behaviour. We instinctively pull away from what we see as a potential threat, and move towards what we see as a potential reward. Each day, thousands of mostly unconscious value judgements are steering us this way and that. It is why we choose to walk on the pavement rather than the road without ever considering the matter.

We are non-stop judging machines, and deciding good or bad is an integral part of any perception. You can test the truth of this right now if you go back to the pairs of opposites in the first sentence of this article. Even though they seem to be nicely balanced intellectual concepts, you will probably find you "like" one side of the pair better than the other. It feels "better" for you than the other, and more deserving of your attention.

Our unthinking assumption that light is "better" than dark, for example, or that day is "better" than night, can lead us astray when both sides of the equation need to be in balance as part of a single dynamic system. Yet we "naturally" prefer the excitement of the day to the unconscious torpor of sleep. We prefer eating to not eating, activity to non-activity, stimulation to boredom, although nothing is automatically good in itself. Any substance, even water, is potentially lethal if taken in excess; it all depends on the dosage. Yet it goes against the grain to realise that too much of any good thing can be toxic.

Some pairs of opposites have a lot of leeway. We can say that every human being has male and female characteristics, but this doesn't mean that the ideal is to be perfectly balanced. We don't really admire the metrosexual or the sensitive new age guy all that much. Our societies profit from the evolutionary gamble of having hyper-masculine men and ultra-feminine women, plus women who are more masculine than the average man, and vice versa.

Our biology is a different matter. Our bodies are always working to maintain an ideal homeostatic balance between a whole range of opposites: the sleep-wake cycle, then tension-relaxation of muscles, the acidity-alkalinity of the blood. Some polarities demand almost perfect balance, and your body will grab your attention immediately if they are just slightly out of whack. Just a few minutes of dangerously low blood pressure or inadequate blood supply to the brain causes death. Other polarities have more wiggle room. Stress is bad for us but it may take years before it strikes us down. This makes it difficult to know how out of balance we can afford to be, or for how long we can postpone paying the bill.

So how much do we need the night? We've beaten it into abject submission in the last century. Night and day no longer balance each other at all in our over-bright, non-stop, over-stimulated societies. Thomas Edison, the inventor of the electric light and patron saint of the 24/7 society, was a compulsively active man and he expected his employees to be the same. He regarded sleep as a dreadful waste of time, and slept very little himself. I once spent three or four years without electricity and I would never, ever want to go back to that lifestyle. Edison was a great benefactor to mankind, and to me, but I know that electricity is not a blessing without a cost.

In the Victorian era before Edison, night lighting came from candles and lamps. Its fuel (beeswax, animal fat or whale oil) was always expensive, so it was used sparingly. As a result, most people couldn't see or do a great deal without sunlight. In fact, until the mastery of fire only a few thousands of years ago, our ancestors had no light at night apart from the moon. We evolved on very familiar terms with the darkness.

As a result, the Victorians probably spent 10 or 12 hours in bed each night for much of the year. Sleep researchers estimate that we sleep on average an hour or two less than they did, which is why sleep deprivation is now so endemic as to be "normal". We are paying the price for too much light.

We need to dream and sleep for psychological health. In the absence of activity and external stimuli, the brain sifts and files the events of the day; it does the housecleaning and throws out the junk; it puts things in perspective and works on outstanding problems, and it prepares us for the following day. It does this crucial work in a way that is impossible while we are awake, and it needs sufficient time commensurate with the volume of the previous day's input.

Nowadays, thanks to Edison's bright inventions, we live, breathe and work in colossal sewers of junk information. We'd have to be saints to avoid it. We are like the rag pickers of Manila, living on mountains of garbage, and our brains have to process every little bit we take in each day.

The average medieval peasant or lord not only had productive sleep and less to think about; he also had an abundance of time, lying half awake in bed, to contemplate and make sense of his life. When did you last lie in bed, in peaceful, boundless reflection? The kind of thinking that occurs in the hypnagogic state between wakefulness and dream can be profound and beautiful, and the source of great intuitions. That state has now largely vanished from our modern mental repertoire, and we're so much poorer for it.

We've now driven the moon into the fringes of our consciousness. When did you last even see the moon or a genuinely dark sky? We live in the triumphant days of a solar society: bright, fast, energetic, active, ever onwards and upwards. We measure happiness by the volume of our consumption. Because we tend to unthinkingly judge one side of a duality as "better" than the other, we don't notice the value of its opposite.

It seems obvious that happiness is better than sadness or despair. Yet, as Rajneesh said, "Life consists of sadness, too. And sadness is also beautiful. It has its own depth, its own delicacy, its own taste. A man who has not known sadness is shallow and even his laughter will lack depth." Furthermore, we need the mental silence and stillness of the night to truly feel our own sadness.

Too much living in the light can dazzle and deceive us. Optimism, positivity and the effort to appear happier than we feel can be self defeating. Sadness and despair are not automatically "bad", or signs of moral failure. They are often appropriate, honest and adaptive responses. They enable us to respond to life and the world as it actually is, without the complicating hazards of impractical fantasy.

Good things really are good, but their neglected opposites are valuable as well. The glamour of hope needs to be balanced by a sense of practical limits. The excitement of action needs the stillness of reflection. We need pain and stress to keep us from harming our bodies. Disappointment tells us when we've done something useless and sets us up for a better course of action.Failing again and again and again are the natural stepping stones to any great achievement (because success without effort is likely to be paltry). Chronic illness can be the inner road to self understanding. Sadness goes hand in hand with the tenderness of love, and gives us a stronger empathy with others than happiness can ever do. Without limitations, inactivity, pain, disappointment, failure, illness and sadness, we are would be less than human. We need to be comfortable with the darkness.